An opt-out agreement is a written agreement you sign to allow yourself to work more than the legal 48-hour average working week. It has to be your own individual choice in writing, not something forced on everyone as part of a standard contract.
You can withdraw it at any time, and your employer can’t dismiss you or treat you badly for refusing to sign one or for withdrawing it.
In practice
You're offered a new contract with a WTR opt-out clause already included. You don't have to sign it. You can cross it out, refuse it, or sign now and withdraw later with notice, and none of that is a reason your employer can use against you.
See also
Legislation covered